1. Field of the Invention
The present invention relates to a battery separator and a manufacturing method thereof, and an alkali secondary battery having the separator incorporated therein, in particular to a nickel-metal hydride secondary battery. More particularly, the present invention relates to a battery separator and a manufacturing method thereof in which preferred hydrophilization treatment is applied, and thus, an alkali secondary battery having the separator incorporated therein causes its superior self-discharge properties; and a nickel-metal hydride secondary battery having the separator incorporated therein, its superior self-discharge properties, and superior charge and discharge cycle life performance.
2. Prior Art
In recent years, with advancement of cordless use, high performance, down-sizing, and reduced weight of various electronic devices such as hand-held telephone or hand-held type personal computers, there has been a growing need for a secondary battery with high-capacity being a power source of these devices.
As power sources of these electronic devices, alkali secondary batteries have been generally employed, and specifically, secondary nickel-cadmium batteries has been mainly used. With a growing demand for the above-mentioned high capacity, recently, secondary nickel-hydrogen batteries are widely used because they are compatible with voltages of the secondary nickel-cadmium batteries, and have higher capacity than the secondary nickel-cadmium battery.
However, there is a problem that, although such secondary nickel-hydrogen batteries have high capacity than the nickel-cadmium batteries, self-discharge is likely to occur if they are in charged state, and maintained under high-temperature environment.
Therefore, corresponding to the fact that use environment of the secondary nickel-hydrogen-batteries is diversified, and these batteries are increasingly used under severe conditions, it is of course that the secondary nickel-hydrogen batteries have high capacity, and it is strongly required that the batteries have improved self-discharge properties and charge and discharge cycle life performance.
In the meantime, an alkali secondary battery is manufactured as follows: In general, a paste consisting of an active substance and a bonding agent is applied to and filled in a collector to manufacture positive and negative electrodes, respectively; an electrode group (generator element) consisting of a separator intervened between the positive and negative electrodes is manufactured; the electrode group is housed in a battery case together with an alkali electrolyte; and then, the battery case is sealed.
For example, in the case of a nickel-metal hydride battery, in general, a separator is intervened between a nickel electrode (a positive electrode) for supporting powders of nickel compounds such as nickel hydroxides and a hydrogen absorbing alloy electrode (a negative electrode) for supporting powders of hydrogen absorbing alloys to form an electrode group; this electrode group is housed in a battery case compatible with a negative electrode terminal together with an alkali electrolyte, and the battery case is sealed.
In such nickel-metal hydride secondary battery, it is of course that the separator must have electric insulation properties, and it is required that the separator has good wettability with an filled alkali electrolyte, whereby it is required to provide electrolyte absorption properties for the alkali electrolyte.
From the foregoing, as a separator, a non-woven fabric consisting of polyamide fibers having good hydrophilicity is widely employed to ensure wettability with the alkali electrolyte.
However, this polyamide fibers are hydrolyzed in an alkali electrolyte, and impurities such as ionic nitrate, nitrous ion, or ammonia, for example, are produced. These impurities cause so-called shuttle reaction, and reduces nickel oxy-hydroxide produced on a nickel electrode during charging, thereby promoting self-discharge in the nickel electrode and degrading battery self-discharge properties.
From the foregoing, as the separator materials, there are attempted selection of fiber materials having oxidization proof superior to the polyamide fibers, for example, polyolefin fibers such as polypropylene fibers, or fluoro resin fibers such as PTFE fibers, and use of which hydrophilization treatment is applied thereto.
Specifically, for example, there is attempted use of a separator in which a non-woven fabric made of polyolefin fibers is surface-treated by an anionic, cationic, or non-ionic surface active agent.
However, the thus manufactured separator ,is not fixed with a hydrophilic group introduced by the surface active agent, which is chemically bonded with a surface of a separator material. Thus, there is a problem that, if battery charging and discharging are repeated, a surface active agent is left from the surface, hydrophilicity are lowered, resulting in lowered charge and discharge cycle life performance.
In addition, the following treatment is attempted: A treatment such as sulphonation treatment or acrylic acid graft treatment is carried out on the full surface of a non-woven fabric made of polyolefin fibers, whereby a hydrophilic active group is added from the outside, and hydrophilicity of the entire surface of the non-woven fabric is enhanced to improve self-discharge properties. Even if these treatments are carried out, self-discharge properties are not sufficiently improved. Moreover, in the case of these treatments, since waste treatment of used chemicals is required, there is a problem that separator manufacturing cost and battery manufacturing cost are increased.
Further, in the case of the nickel-metal hydride secondary battery, it is known that self-discharge is promoted by hydrogen being left from a hydrogen absorbing alloy used for a negative electrode. The mechanism is described as follows:
When a nickel-metal hydride secondary battery is stored under a high temperature, an equilibrium pressure of a hydrogen absorbing alloy for a negative electrode rises together with a temperature rise, and a quantity of hydrogen capable of being absorbed by the negative electrode decreases. As a result, hydrogen gas which cannot be accumulated by the negative electrode is discharged into a battery. This hydrogen permeates the separator, reaches a surface of a nickel electrode, reduces a nickel oxy-hydroxide being a charged product of the nickel electrode, and causes self-discharge.
Thus, requirements for improvement of self-discharge properties of an alkali secondary battery, in particular, a nickel-metal hydride secondary battery are that a separator to be incorporated is composed of materials without producing the aforementioned impurities, and that high hydrophilicity are imparted, thus making it possible to ensure wettability with an alkali electrolyte over a long period of time.
On the other hand, when electric conductivity between active substances (nickel hydroxides) in the aforementioned nickel electrode and between an active substance and a collector is increased, the utility of the active substance is increased. Also, since a firm electrically conductive matrix is formed at the surface of nickel hydroxide powder, self-reducibility of the nickel electrode per se is suppressed. In this manner, the charge and discharge cycle life performance of the secondary nickel-hydrogen batteries having the nickel electrode incorporated herein are improved.
Therefore, conventionally, the following method is carried out for the purpose of increasing the utility of the active substance and improving the charge and discharge cycle life performance.
For example, when a positive electrode synthetic agent to be filled in or applied to a nickel electrode is paste-prepared, cobalt compounds such as cobalt metals, cobalt hydroxides, cobalt tri-oxides, tri-cobalt tetra-oxides, or cobalt monoxides or a predetermined amount of particles of a mixture of these compounds are added as an electric conducting material, mixed powders mixed with powders of nickel hydroxides at a predetermined rate is produced, and the produced powders are used as an active substance.
When a nickel electrode on which the thus produced active substance powders are supported is incorporated as an alkali secondary battery (a nickel-metal hydride secondary battery), a cobalt metal or cobalt compound contained in the above mentioned active substance powders is temporarily dissolved in an alkali electrolyte to be a complex ion, and the complex ion covers a surface of nickel oxide powders. Further, during initial battery charging, these complexions are oxidized prior to nickel hydroxides; the oxidized ion is converted to a higher-order oxide consisting essentially of an electric conducting cobalt oxy-hydroxides; the converted oxide is deposited between nickel oxide powders being an active substance and between an active substance layer and a collector; and a so-called electric conducting matrix is formed. As a result, electric conductivity between active substances and between an active substance and a collector is improved, whereby the utility of active substances is improved.
In addition, it is also known that, while the aforementioned metal cobalt or cobalt compound and nickel oxide powders are mixed with each other at a predetermined rate under an oxygen-containing atmosphere, a predetermined amount of alkali aquous solution is added herein at the same time, and the entirety is irradiated with microwaves, for example, and is heated uniformly at a predetermined temperature, thereby producing an active substance.
In this case, a part of a cobalt metal or cobalt compound is dissolved in the added alkali aquous solution to be a complex ion, the complex ion covers a surface of nickel oxide powders, and at the same time, is converted to a higher-order cobalt oxide. Thus, an active substance to which the above treatment has been applied has a higher-order cobalt oxide layer already formed on the nickel oxide powder surface being an intrinsic active substance.
It is one object of the present invention to provide a battery separator having its superior hydrophilicity and a manufacturing method thereof without producing impurities that cause the above mentioned shuttle reaction when the separator is incorporated in a battery.
It is another object of the present invention to provide an alkali secondary battery having its superior self-discharge properties, in particular, a nickel-metal hydride secondary battery by incorporating the above mentioned separator.
It is a further object of the present invention to provide a nickel-metal hydride secondary battery having both of its improved self-discharge properties and charge and discharge cycle life performance.
To achieve the foregoing objects, according to the present invention, there is provided a battery separator consisting of: synthetic resin fibers, wherein hydrophilization treatment is applied, and a contact angle relative to a pure water indicates 0 to 100 degrees. Preferably, there is provided a battery separator, wherein the hydrophilization treatment is carried by plasma treatment, an area with a certain difference in hydrophilicity is formed in a surface or thickness direction, an material thereof is a non-woven fabric consisting of a polyolefin based synthetic resin fibers, a specific surface area for the non-woven fabric is 0.5 to 5.0 m2/g, plasma treatment using O2 gas is further applied to the separator, and when ESCA (Electron Spectroscopy for Chemical Analysis) measurement is carried out, the O/C ratio in a site of at least 20 xc3x85 in depth is 0.01 to 0.6.
In addition, according to the present invention, there is provided a manufacturing method of the battery separator comprising: arranging a separator material consisting of synthetic resin fibers on the earth electrode of a plasma treatment apparatus wherein a power electrode and an earth electrode are arranged in parallel; and performing plasma treatment for the separator in an atmosphere containing at least gas for imparting hydrophilicity.
In particular, there is provided a manufacturing method of battery separator, wherein the aforementioned atmosphere is an atmosphere of a mixture gas between a gas for imparting hydrophilicity and a gas for imparting hydrophobicity, and the aforementioned plasma treatment is a plasma treatment carried out in the gas for imparting hydrophobicity after being carried out in the gas for imparting hydrophilicity or carried out in the gas for imparting hydrophilicity after being carried out in the gas for imparting hydrophobicity; the plasma treatment is carried out in a state in which a surface of the aforementioned earth electrode is not exposed to the aforementioned atmosphere; the aforementioned gas for imparting hydrophilicity is at least one selected from the group consisting of oxygen, nitrogen, air, nitrogen oxides, ammonia and carbon dioxide; and the aforementioned gas for imparting hydrophobicity is at least one selected from the group consisting of tetra-fluorinated carbon, tetra-fluorinated ethylene, and hexa-fluorinated ethane.
Further, according to the present invention, there is provided an alkali secondary battery comprising: electrode groups having the separator intervened between positive and negative electrodes being sealed in a battery case together with an alkali electrolyte.
In particular, according to the present invention, there is provided a nickel-metal hydride secondary battery comprising: the electrode groups having the separator intervened between a nickel electrode and a hydrogen absorbing alloy electrode being sealed in a battery case together with an alkali electrolyte, an active substance of the nickel electrode comprising powders consisting essentially of nickel hydroxides and higher-order oxides of cobalt formed on a surface of a part or whole thereof.